[GHC] #15650: Add (or document if already exist) ability to derive custom typeclasses via source plugins

[GHC]

#15650: Add (or document if already exist) ability to derive custom typeclasses via
source plugins
-------------------------------------+-------------------------------------
        Reporter:  chshersh          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:  source
                                     |  plugins,deriving,typeclass
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12457            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #12457


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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15650#comment:2>
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